1. Field of the Invention
The present invention relates to a coupled line, and more particularly, to a coupled line for use at high frequencies.
2. Description of the Related Art
FIG. 4 is a cross-sectional view of a conventional coupled line including microstrip lines. Referring to FIG. 4, a coupled line 20 comprises a dielectric substrate 21, microstrip lines 22 and 23, and a ground electrode 24. The microstrip lines 22 and 23 have a rectangular cross-section, are arranged with a space g3 therebetween, and are electromagnetically coupled to each other.
However, in the aforesaid prior art, it is impossible to make the space between two microstrip lines smaller than the smallest space which is obtainable by a conventional electrode forming process, because light enters the area under the photomask which should remain unexposed, to produce the space between the two microstrip lines, when the resist adjacent to the photomask is exposed. Accordingly, the area under the photomask is also exposed and it is impossible to produce the space. Therefore, the degree of coupling between the microstrip lines, which depends on the space therebetween, is limited by the electrode forming process.